1. Field of the Invention
The present invention relates to an electrophotographic photoconductor with high durability which is capable of realizing high quality of images for a long period of time. Also, it relates to an image forming process, an image forming apparatus and a process cartridge for an image forming apparatus using the long life and high performance photoconductor.
2. Description of the Related Art
Recently, the organic photoconductor (OPC) is widely used in a copying machine, facsimile, laser printer and a composite thereof owing to excellent performance and various advantages, instead of the inorganic photoconductor. The reason includes, for example, (1) optical properties such as range of light absorbing wave length and absorption amount, (2) electrical properties such as high sensitivity, stable charging properties, (3) width of selection range of materials, (4) easiness of preparation, (5) low cost, (6) non-toxicity and the like.
Meanwhile, as the image forming apparatus gets smaller, a photoconductor with smaller diameter is also sought. Further, tendency of high speed and maintenance free is added and thus there is great demand for high durability of the photoconductor. In this point of view, the organic photoconductor has a defect in that when it is repeatedly used in the electrophotographic process, it is susceptible to abrasion by mechanical load of a developing system or a cleaning system since the surface layer comprises mainly a low molecular charge transport material and an inactive high molecule (polymer) which are generally soft. Also, due to the demand for high image quality along with small diameter of toner particles, increase in rubber hardness and increase in contact pressure of a cleaning blade to enhance cleaning property are forcedly required, which is another factor to promote the abrasion of the photoconductor. Such abrasion of the photoconductor leads deterioration of electrical properties such as sensitivity and chargeability and thereby, deteriorated image with reduction of image density and contamination of the ground surface. Also, a damaged part with local abrasion produces a contaminated image with a striped pattern by cleaning failure. At this moment, the life span of the photoconductor is determined by the abrasion and damage.
Therefore, it is necessary to reduce the above-described abrasion in order to increase durability of the organic photoconductor and this is the most urged matter to be solved in the art.
The techniques to improve abrasion resistance of the photoconductive layer include (1) using a curable binder in the surface layer (for example, JP-A No. 56-48637), (2) using a high molecular charge transport material (for example, JP-A No. 64-1728), (3) dispersing an inorganic filler in the surface layer (for example, JP-A No. 4-281461) and the like. Among these techniques, the use of a curable binder in (1) tends to cause reduction in image density since the curable binder has poor compatibility with the charge transporting material and impurities such as a polymerization initiator and unreacted residue increases residual potential. Also, the use of a high molecular charge transport material in (2) may somewhat improve the abrasion resistance. However, it is not sufficient for satisfying the durability required in the organic photoconductor. Further, it is difficult to polymerize and purify the high molecular (polymer) charge transporting material. Thus, it is impossible to obtain it at a high purity and to attain stable electrical properties between materials upon using it. In addition, it may cause problems such as high viscosity of the coating solution in terms of the preparation. The dispersion of the inorganic filler in (3) shows high abrasion resistance, as compared to that of the conventional photoconductor comprising a low molecular charge transporting material dispersed in inactive high molecules (polymer). However, traps on the surface of the inorganic filler tends to increase the residual potential, thereby causing reduction in the image density. Also, when unevenness of the inorganic filler and the binder resin on the surface of the photoconductor is severe, cleaning failure may occur, resulting in toner peeling and image deletion. With these techniques of (1), (2) and (3), it is impossible to satisfy the durability required for the organic photoconductor, including electrical durability and mechanical durability.
Also, in order to improve electrical properties of (1), JP-A No. 2002-6526 discloses a technique of a protective layer containing an electroconductive filler. The photoconductor used in this technique may inhibit increase of residual potential by repeated use. However, it has defects in that since resistance of the protective layer decreases in a high humidity circumstance, reduction of resolution and image deletion may occur.
Furthermore, in order to improve the abrasion resistance of (1) and scratch resistance, a photoconductor containing a cured body of a multi-functional acrylate monomer is disclosed (Japanese Patent No. 3262488). In this patent, the purpose of inclusion of a cured body of this multi-functional acrylate monomer in a protective layer on the photoconductive layer is described. However, whether a charge transporting material may be contained in the protective layer is only described without concrete description. Further, when a low molecular charge transport material is simply added to the surface layer, it may cause problems related with the compatibility to the cured body, whereby crystallization of the low molecular charge transporting material and clouding may occur, resulting in reduction in mechanical properties.
In addition, according to this photoconductor, since the monomer is reacted while it contains a high molecular binder, the curing cannot be sufficiently progressed. Also, the cured body is poorly compatible with the binder resin and surface unevenness by phase separation upon curing may occur, causing cleaning failure.
As technique for inhibiting abrasion of the photoconductive layer to substitute the above techniques, a process for forming a charge transporting layer using a coating solution comprising a monomer having carbon-carbon double bond, a charge transport material having a carbon-carbon double bond and a binder resin (for example, Japanese Patent No. 3194392). The binder resin includes a binder reactive with the charge transport material having a carbon-carbon double bond and a binder non-reactive with the charge transport material without having the double bond. This photoconductor has attracted public attention since it shows abrasion resistance along with excellent electrical properties. However, when a non-reactive resin is used as the binder resin, the binder resin is poorly compatible with the cured body produced by the reaction of the monomer and the charge transport material, whereby surface unevenness during cross-linking forms from the phase separation, resulting in cleaning failure. Also, as described above, in addition to the interference of the binder resin with the curing of the monomer, a bi-functional monomer which can be used in the photoconductor has a few functionality and fails to provide a sufficient cross-linkage density, whereby it is possible to obtain a sufficient abrasion resistance. Also, when a reactive binder is used, since the number of functional groups contained in the monomer and the binder resin is small, the bonding of the charge transporting material and the cross-linkage density cannot be satisfied at the same time and the electrical properties and abrasion resistance are not sufficient.
Also, a photoconductive layer containing a hole transporting compound curing a compound having two or more chain polymerizable functional group in a molecule (for example, JP-A No. 2000-66425).
However, according to the photoconductive layer, since a big hole transporting compound has two or more chain polymerizable functional group, distortion may occur in a cured body, causing increase in internal stress, roughness of the surface layer and formation of crack over the time.
Even in a photoconductor having a cross-linked photoconductive layer with a charge transporting structure chemically attached, it cannot be said that general properties are sufficiently attained.